Strike Witches is set in an alternate version of 1944, where the events of World War II are very different than what we know to be true. Alien invaders known as the Neuroi came to Earth in 1939. This is not the first appearance of the Neuroi as they have appeared at seemingly random intervals throughout human history. While the Neuroi primarily attack humanity using aircrafts, their mothership is able to produce a deadly miasma, a chemical that is fatally poisonous to humans, forcing the populations of affected areas to flee their homes. The Neuroi then use the poisoned areas to plunder Earth's natural resources to use against humanity. Luckily the miasma cannot spread across water, making the ocean humanity's main line of defense against the Neuroi.